1. Field of the Invention
This invention relates to methods for testing aldehyde in polyester polymers using an aldehyde-sensitive indicator. The inventive indicator is capable of detecting acetaldehyde (AA) in the microgram range.
2. Summary of the Prior Art
During melt molding of polyethylene terephthalate (PET), such as injection molding a preform and extrusion molding a parison for packaging containers, AA is generated as a by-product due to the thermal degradation of the PET polymer. The AA formed is trapped within the molded preform or parison and packaging container made therefrom. Ultimately, the entrained AA diffuses gradually from the container wall into the liquid/beverage contained therein. If present in sufficient quantities, the migrated AA can impart a sweet fruity-like odor and flavor to the product, thereby affecting its perceived quality. The sensory detection threshold of AA in water has been reported to be 20 to 40 μg/L levels in water. The odor detection thresholds in carbonated soft drinks are expected to be much higher because of the masking effect of other ingredients.
The amount of AA generated depends greatly on the heating time and temperature experienced by the polymer during the molding process. Accordingly, AA testing is an important quality assurance procedure for PET preforms/bottles manufacturing to ensure that the finished containers are within the acceptable AA specification.
Currently, two methods are used in the industry for determining AA in PET: ground parison (GPAA) and headspace (HSAA) methods. The GPAA method entails cooling preforms or resin pellets in liquid nitrogen and grinding them in a mill to form powders (typically less than 1 mm in size). The PET powder is then weighed into an airtight headspace vial and heated to 140-160° C. for 60-90 min. Following the heating process, the AA collected in the headspace is sampled and analyzed using a gas chromatograph (GC). The concentration of AA in the PET is typically expressed as ppm (one part AA per one million part PET by weight). Typical acceptable GPAA values can range 8-25 ppm of PET.
The HSAA method involves measuring the amount of AA diffused into the headspace of an unfilled bottle after conditioning the bottle at 22-25° C. for 24 h after blow molding a preform to form a bottle. The AA concentration of the headspace air is determined by sampling a portion of the bottle headspace and analyzing the headspace sample with a GC. This is typically performed automatically using an auto-sampling system. A typical upper limit of GPAA value is in the range of 4-5 mg/L AA concentration of the bottle headspace.
The GPAA method measures residual AA in the PET preform, and thus is a direct measurement of AA content in the PET sample. The HSAA method provides only an indication on the AA content for the PET bottle, as the AA test results which are dependent on the size/shape, stretch ratio, crystallinity, conditioning time/temperature and other parameters for the container. Preform makers, blow molders and end users have established GPAA and HSAA values that should not be exceeded to assure that the residual AA does not affect the perceived quality of the liquid product significantly. Although the HSAA and GPAA methods are currently accepted by the industry for evaluating AA in PET, these tests are laborious and time-consuming. Moreover, the test instruments are expensive, complicated, and require a specially trained individual to operate the equipment.
In search of a faster, easier and more cost-effective method, the inventors herein have developed the disclosed aldehyde indicator techniques. Indicator strips have been used extensively for detecting and quantifying compounds in various areas ranging from clinical fields to food industries. In general, these tests are characterized by their simplicity, which consists of exposing an indicator strip to the test sample and reading the results. These indicators are, in general, made of an adsorbent or carrier impregnated with the reagents that are sensitive to the compound of interest. There have been several methods and related procedures developed for determining aldehydes. Each of these known methods contains some disadvantage causing them to be less than optimal. For example, U.S. Pat. No. 4,511,658 describes a method of applying a ketone solution of 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole (AHMT, trademark name “Purpald” from Aldrich Chemical Co. Milwaukee, Wis.) on inert solid support coated with a metal bicarbonate (e.g., NaHCO3 or KHCO3). The colorimetric detector is said to be highly selective to formaldehyde, and therefore precludes its use for testing other aldehydes, including AA. U.S. Pat. No. 5,128,171 disclosed a methodology for preparing a test strip comprises a support and a reagent layer consisting of a dialyzed latex polymer (e.g., carboxylated vinyl acetate/ethylene copolymer), 3-methyl-2-benzothiazolinone hydrazone (MBTH) and other compounds. It claims that by using a dialyzed polymer in the reagent layer, the resulting test strip maintains its integrity for a substantially longer time as compared to using an undialyzed polymer. While the use of this strip may be suitable for testing liquid samples, its use in testing AA in air may be less effective because AA needs to permeate into the latex matrix and react with the reagents contained therein. U.S. Pat. Nos. 3,645,696 and 3,784,358 describe methods to stabilize a chromogenic reagent for use in the indication of ethylene glycol antifreeze contamination in crankcase oil. The patents describe applying MBTH on solid supports (silica gel or porous polyethylene) and drying in nitrogen gas. This method is intended for testing aldehydes in oil and/or liquid medium, but it is not optimal for testing AA in air. In addition, when a transmission measurement of the colored solution is used for achieving the desired test sensitivity, the chromogen formed on the solid support tends not to dissolve easily in the oxidizing solution. Another U.S. Pat. No. 4,438,206 discloses a method to improve the test sensitivity of acetylacetone by adding a serum albumin to the solution. However, as noted by the inventors, the enhancement of fluorescence intensity by addition of the serum albumin is achievable only in the reaction of the acetylacetone with formaldehyde, but not AA.
Many commercial products are also available for testing aldehyde compounds in air which involve passing the test air through a cartridge, tube or badge containing aldehyde-sensitive reagents. These devices require a large volume of air, long exposure time, or further desorption and separation treatment upon reaction, which render them unsuitable for testing AA in limited headspace air for PET samples.
In a PET preform/bottle production environment, it is important to identify quickly a deviated process as it occurs to reduce scrap and minimize the production cost. Therefore, it is of great benefit to have an AA test method that can be performed routinely, rapidly, cost-effectively and simply enough for use by the operator of a molding machine without involving a complicated analytical technique. This invention describes such a test process involving the aldehyde-indicator technique for quantitative and semi-quantitative testing of AA in PET polymer.